Hitachi's U.S. Pat. No. 5,569,988, incorporated herein by reference, discloses a motor driving circuit for driving a three-phase brushless spindle motor of a hard disk drive (HDD). The disks of the HDD are rotated by the spindle motor. Variations on the Hitachi circuit may be found in USPP 2004/0108827, also incorporated herein by reference.
Feedback from the motor output has been used to establish a sinusoidal input current to the motor. In such an implementation, because the pattern of the three-phase output does not switch the current-supply pattern suddenly, but “softly” within predetermined phase criteria, it is colloquially referred to as a “soft switch”.
In any case, the effect of such a control circuit is salutary, because it reduces the acoustic noise that is produced. As critically recognized herein, however, when the HDD is operating at relatively cold temperatures, such as at start-up, the soft switch feature can lead to undesirable speed oscillations of the spindle motor. With this critical recognition in mind, the invention herein is provided.